The telecommunications industry is experiencing a remarkable growth in cellular mobile telephony as well as Internet use. One of the results of this growth is the increasing use of cellular wireless data services. Thus, the need to support mobility is no longer limited to voice service but also extends to data service.
Within the enterprise there is a growing use of wireless local area networks (WLANs). Currently WLANs are primarily used for data, even though voice can also be supported. A major effort is under way to provide seamless roaming between the cellular data network and the WLAN. This will permit data applications, such as a Transmission Control Protocol (TCP) based downloads or User Datagram Protocol (UDP) based streaming video, that originate when the user is using his/her laptop or mobile device in the WLAN to continue uninterrupted when the user moves outside the limits of the WLAN into the cellular data network, such as the General Packet Radio Service (GPRS) network. Similarly, it will enable a connection that originates in a cellular data network, which is hereafter referred to as a wireless wide area network (WWAN), to continue uninterrupted in a WLAN when the user roams into a building with poor or no WWAN signal reception.
Some current data handoff schemes are based on Mobile IP, which works through the following three elements:
Mobile Node, which is a node that has a permanent IP address registered in its “home” network. The node can change its point of attachment to the Internet while being reachable through the same IP address.
Home Agent, which is router that serves the mobile nodes in a given subnet. When a Mobile Node connects to a foreign network, which is any network that is not its home network, it registers its care-of address (COA) with the Home Agent. The Home Agent uses the COA to forward packets arriving at the home network and destined for the Mobile Node to the node. This packet forwarding is done via a technique called tunneling.
Foreign Agent, which is a router in the visited (or foreign) network whose IP address the Mobile Node uses as its COA. The tunnel from the Home Agent terminates at the Foreign Agent, and it is the responsibility of the Foreign Agent to forward packets arriving via the tunnel to the Mobile Node.
These three elements interact through the following three processes:
Agent discovery is the process by which the Foreign Agent advertises its availability on its network to the roaming Mobile Nodes. A Foreign Agent periodically broadcasts the agent discovery message, which enables the Mobile Nodes to obtain their COA. If a Mobile Node does not receive the agent discovery message within some predefined time interval after it has connected to the subnet, it broadcasts an agent solicitation message that asks the available Foreign Agents in the subnet to identify themselves to the Mobile Node.
Registration, which is the process by which a Mobile Node in a foreign network requests service from the Foreign Agent and registers its COA with the Home Agent.
Routing, which is the process by which packets are routed to and from a Mobile Node in a foreign network.
Mobile IP is defined in the Internet Engineering Task Force (IETF) RFC 2002 for IPv4 and updated in RFC 3344. Mobile IP for IPv6 is currently in the draft stage. Mobile IP permits a Mobile Node to roam from one network to another and still be able to maintain connectivity with the home network. However, this requires the node to physically connect to the visited network first before it can communicate with the home network. Thus, it does not necessarily provide a fast and seamless handoff between the current network and the next network to be visited. This means that a Mobile Node may experience data loss and delay that can be detrimental to real-time multimedia applications that use reliable transport protocols.
Also, Mobile IPv4, which is currently used in most networks, is based on the idea that a Mobile Node has a permanent IP address through which it can be reached anywhere in the world. However, with the shortage of IPv4 addresses and the prevalent use of private (or non-routable) IP addresses and network address translation (NAT), most networks cannot support Mobile IP in a clean way.
One scheme that is being developed within IETF to address the limitation of requiring a Mobile Node to have a permanent IP address is the Diameter protocol. The protocol permits a Mobile Node to use its network access identifier (NAI), which is the user ID submitted by a Point-to-Point Protocol (PPP) client (such as a laptop) during PPP authentication. Diameter extensions for Mobile IPv4 have been proposed to allow a MIPv4 node to receive services from service providers (home and foreign) and allowing the Diameter servers to authenticate, authorize and collect accounting information for those MIPv4 nodes. Similarly, Diameter extensions to Mobile IPv6 have been proposed since the architecture of Mobile IPv4 is slightly different from that of Mobile IPv4. However, while the Diameter protocol may resolve the issue of static IP address assignment, it does not support dynamic data handoff in the sense that a Mobile Node cannot transfer a connection created in one network to another network.
Another set of protocols that have been proposed deals with TCP over heterogeneous networks (ToHN). The goal of these protocols is to improve the performance of the TCP connections over wireless links. However, these protocols, which include indirect TCP, are fixed protocols that assume that the TCP server and the TCP client are in wired and wireless networks respectively, or vice versa. They do not deal with dynamic handoff when the client moves from one network to another.